This invention relates to electromagnetic transmission and reception systems, and relates specifically to the transmission and reception of signals in the Terahertz (THz) radiation band.
Terahertz radiation represents the last band of the radio wave and light spectrum which has not been extensively used for communications systems and other applications. Until recently, bright sources of light and sensors have not been available for this frequency band. The Terahertz band is generally considered to cover the range from 100 GHz (1011 Hz) up to roughly 30 THz (3×1013 Hz), and this corresponds to wavelengths from approximately 3 mm to sub-millimeter wavelengths of approximately 0.01 mm. The use of the term Terhertz in this application is intended to cover this range.
The lower frequency limit lies just above the microwave region where satellite dishes and mobile phones operate, whereas the upper limit is located adjacent to infrared frequencies used in devices such as television remote controllers. Conventional microwave sources do not operate at sufficiently high frequency to efficiently produce radiation in the gap, whereas laser diode sources have been limited by thermal effects.
Two “laser-based” methods of THz generation have recently been developed. Some development has taken place for “true” THz lasers, which involves making materials which can emit at these frequencies (this is at a very primitive stage). The use of ultra-short pulses generated in optical/infrared lasers is, however, more advanced. These pulses are then detected in semiconductor (often cooled) devices and then radiated. This results in low power sources with a broad spectral content up to THz frequencies, providing sufficiently short pulses are used.
The use of Terahertz frequency light is particularly suitable for a number of applications, such as the imaging and chemical analysis of a variety of objects, including human tissue for potential medical applications. Non-medical applications include security screening and non-destructive testing, as materials such as plastics, clothing, cardboard, and semiconductors are transparent to Terahertz radiation.
For imaging applications, these millimeter and sub-millimeter wavelengths allow for the distinction between a wide variety of material features, and also offer good resolution.
Conventional imaging at these frequencies only detects the intensity of the reflected signal, by a detector or detector array. Methods to decrease the coherence level are regarded as essential in order to achieve good images, such methods including polarisation diversity, and random pattern creation (e.g. from coarse reflectors). A coherent source gives a complex amplitude function and it is then appropriate to detect the reflected signal using coherent detection, wherein both the intensity and the phase of the detected signal are recovered. Accurate control of the frequency is also desired to provide a better spectral resolution resulting in increased information content for the images.
The Terahertz frequency band can also potentially be used in wireless communications systems, particularly line of sight communications. In wireless communications applications, moving to higher frequencies can provide more bandwidth and hence increased data transmission speeds. However, in such a system, which would typically be fed from an optical fibre distribution network, the data rate per channel is limited by the chromatic dispersion in the fibre.
This invention relates particularly to a generator and coherent receiver for signals including signals in the Terahertz band. Existing light sources for this band, as described above, are relatively broad spectral sources, and thus offer weak power spectral density and unstable frequency of illumination. For a coherent imaging technique, a much more stable source is required and with a high spectral density of the illumination, in order to reduce the required overall signal to noise ratio.